The invention relates to a method for the closed-loop control of the quantity of dampening solution during printing, in which the metering of the quantity of dampening solution to be applied to the printing material is preferably selected as a function of the printing speed.
Methods for the closed-loop control of the metering of dampening solution, in particular during offset printing, on the basis of the measured quantity of dampening solution, for example on the printing plate having ink-free and printed areas, are known. In this case, the measured variable used is, for example, the layer thickness of the dampening solution, measured by infrared absorption, the gloss of the printing-plate surface, which changes with increasing layer thickness, or the like.
U.S. Pat. No. 5,050,994 (German patent DE 38 30 732 C2) discloses a method of supplying a dampening solution in an offset printing machine. There, using an opto-electronic converter, measured brightness values are obtained from a printed original having ink-free and printed areas, and in which the measured brightness values are used to derive signals for actuating elements with the aid of which the quantity of dampening solution supplied to a printing plate is led to a desired value. According to the method, the signals produced by scanning the areas lying behind the edges of the predefined inked areas are compared with comparative values. Depending on the result of the comparison, a dampening deficiency signal is derived, which identifies an excessively low dampening solution feed. The scanning can preferably be carried out on a printed sheet but scanning on the rubber blanket or on the printing plate clamped in is not ruled out.
German patent application DE 197 01 219 A1 discloses a method for the open-loop control of the inking during printing, in which the metering of the fluid to be applied to a printing material is varied as a function of the printing speed, and in the event of a change in the printing speed, the fluid is applied in controlled excess for some time at the same time as the change in the printing speed.
German patent application DE 41 22 794 A1 discloses a method for the monitoring and closed-loop control of the printing process, in particular on offset printing machines, in which, for example, tonal value gains are not controlled exclusively by varying the ink feed. The method according to the invention is in this case organized into a learning and a doing phase. The quality features which can be registered on the printed product by measurement form a feature space in which membership vectors from printing trials selected in accordance with the method are assigned to specific classes and/or causes for printing disruptions (learning phase). In the doing phase, that is to say the closed-loop control and monitoring of the printing process, an indication is given on the basis of the determined membership of the class and/or the cause of the printing disruption in this class, and/or precisely via the cause determined in this way, action is taken in the process with the objective of eliminating this cause.
The fundamental objective of the printing process is the optimum transfer of ink in the printing area of the plate and keeping it free in the non-printing area. In manual terms, this state is adjusted by moving to the so-called smearing limit, that is to say the start of scumming, and slight overdampening, usually 10%. However, there is no method for the on-line registration by measurement of the state relating to the smearing limit. In order nevertheless to be able to implement dampening solution closed-loop control, the variable which is indirectly related thereto, the dampening film thickness or the like, is measured and regulated via the dampening solution metering. In general, the quality of closed-loop control is restricted considerably and is inadequate.
However, the ink-water balance as referred to the entire process is critical for the quality of the printed image. In this case, in addition to the quantity of dampening solution on the plate or the printed original, the proportion of water in the ink and the surface water on the applicator rolls in particular play an important part.
It is accordingly an object of the invention to provide a method of controlling the dampening in a printing unit, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for closed-loop control in which a deficiency of dampening solution and an excess of dampening solution are controlled out, taking account of the ink-water balance as refers to the entire process.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the closed-loop control of a quantity of dampening solution in a printing process, which comprises:
metering a quantity of dampening solution to be applied to a printing material in the printing process and adjusting the quantity in dependence on a first variable (preferably the printing speed);
in addition to the first variable, taking into account at least one further variable influencing the requisite quantity of dampening solution in the printing process in the form of a set-point change xcex94W as an input parameter for a dampening-solution controller.
In accordance with an added feature of the invention, the at least one variable that influences the printing process is selected from the group of the printing speed, the temperature of an inking unit, the temperature of an ink fountain, the temperature of the ink, the type and the state of the printing plate, the number of prints made, the ink density, the elapsed printing time, the water absorption capacity of the ink, the concentration of dampening solution additives, and the ink strip width.
In accordance with an additional feature of the invention, the (at least one) further variable influencing the quantity of dampening solution in the printing process is defined with a measurement during the printing process, i.e., the variable is a real-time variable.
In accordance with a concomitant feature of the invention, the set-point change xcex94W is defined as a polynomial expression in n variables up to an order q. The numbers n and q are natural numbers, and n specifies the number of variables being taken into account.
The ink-water balance is influenced by various factors. In addition to the printing speed, the set of influencing variables includes the printing speed, the temperature of the inking unit, the temperature of the ink fountain, the temperature of the ink, the type and the state of the printing plate, the number of prints already made with the present printing plate, the ink density, the elapsed printing time, the water absorption capacity of the ink, the concentration of the dampening solution additives. These influencing variables can to some extent be measured very easily or are already present as variables in the machine control system. The method according to the invention for the closed-loop control of the quantity of dampening solution during printing, in which the metering of the quantity of dampening solution to be applied to the printing material is preferably selected as a function of the printing speed, comprises taking account of at least one variable that influences the requisite quantity of dampening solution in the printing process. In this case these may be in particular variables that influence the printing process from the set of different influencing factors indicated above. In particular, the dampening solution metering can be varied when there is a variation in the printing speed. For the measured quantity of dampening solution Y, for a predefined, substantially fixed set of influencing factors, that is to say certain parameters, a set point W is predefined and the inclusion of the relevant influencing factors, that is to say taking account of the deviations or variations of relevant influencing factors in relation to the respectively corresponding values in the predefined, substantially fixed set of influencing factors for the set point W, is carried out in the form of a set-point change xcex94W In this case, the set-point change is a function of the n variables that are taken into account and influence the quantity of dampening solution in the printing process, n being a natural number, in addition to the actual quantity of dampening solution. The functional relationship for the set-point change xcex94W can advantageously be represented around the point of the set point W, in other words an operating point, as a series expansion up to an order q, with q taken from the natural numbers. In the simplest case, the set-point change xcex94W can be described as a linear polynomial, that is to say an expression of first order, in the form
xcex94W=A0+(A1xc3x97Z1)+(A2xc3x97Z2)+ . . . +(Anxc3x97Zn)
where the variables Zi, i=1, . . . , n, n being a natural number, describe the n influencing factors and are to be understood as deviations from the value at the operating point, and A0 and Ai, i=1, . . . , n are constant coefficients. In general, the coefficients can be determined by means of printing trials or calculation on the basis of an appropriate model. More complex relationships, in particular in the event of interaction between the individual variables, can if necessary be taken into account by a polynomial expression of higher order or a similar mathematical relationship. As an example in one advantageous embodiment of the invention mention should be made of the quadratic polynomial expression, that is to say of second order,       Δ    ⁢          xe2x80x83        ⁢    W    =            A      0        +                  ∑                  i          =          1                n            ⁢                        A          i                xc3x97                  Z          i                      +                  ∑                  j          ,                      k            =            1                          n            ⁢                        A          jk                xc3x97                  Z          j                xc3x97                  Z          k                    
where A0, Ai and Ajk, i, j, k=1, . . . , n are constant coefficients. In particular for the linear case, the printing speed, the temperature and the ink density, that is to say dampening solution tracking in the event of an inking change, can be taken into account. For the case of a higher order than a first order, it should be ensured that the series expansion in the n influencing factors converges, and that the expansion is consistent in all the variables Zi, i=1, . . . n taken into account.
The method according to the invention has significant advantages. As a result of registering by measurement different variables that influence the quantity of dampening solution in the printing process, that is to say the ink-water balance, on-line registration of the margin from the smearing limit is made possible, so that input parameters for a closed-loop control system can be made available. The control quality can be increased. Since, for substantial parameter ranges, there is a linear relationship between the quantity of dampening solution and at least one of the variables that influence it, it is possible to have recourse to the standard methods for multi-parameter control systems which are known from control engineering. For example, such standard methods are described by H. Unbehauen xe2x80x9cRegelungstechnik IIxe2x80x9d [Control Engineering II], Vieweg Verlag, Brunswick/Wiesbaden 1997. Advantageously, in a development of the invention, a distinction can be drawn between variables that have a high and low influence on the quantity of dampening solution in the printing process whose corrections can be taken into account up to different orders even in each case independently of one another.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a dampening control method taking into account several variables influencing the printing process, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of the specific embodiment when read in connection with the accompanying drawing.